1. Field of the Invention
The present invention relates generally to electronic circuit assemblies, and more particularly to a method for making an electronic circuit assembly out of etched trimetal-layered circuit structures.
2. Disclosure Information
U.S. Pat. Nos. 3,801,388 to Akiyama et al. (hereinafter xe2x80x9cAkiyamaxe2x80x9d), U.S. Pat. No. 4,404,059 to Livshits et al. (hereinafter xe2x80x9cLivshitsxe2x80x9d), and U.S. Pat. No. 5,738,79/7 to Belke, Jr. et al. (hereinafter xe2x80x9cBelkexe2x80x9d), all of which are incorporated herein by reference, disclose various methods for making electronic circuits which feature circuit crossovers or xe2x80x9cair bridgesxe2x80x9d using a combination of plating and etching steps.
The air bridge structures disclosed in these patents are useful in offering design flexibility and printed circuit board real estate savings as far as routing and layout of circuit traces; however, they do not disclose or suggest any approach for accommodating certain circuit board interconnect processes, such as heavy wirebonding (e.g., using 5- to 20-mil aluminum wire, such as in wirebonding power transistor dice to leadframes or mounting pads), fine wirebonding (e.g., using less-than-5-mil gold wire, such as in connecting the I/O pads of bare integrated circuit dice to their respective circuit board mounting pads), or direct component attachment (e.g., bonding of flip-chips, BGAs (ball grid arrays), and the like directly to traces/pads on a circuit board substrate).
It would be desirable, therefore, to provide a method for using the aforementioned air bridge circuit structure with such interconnect processes as heavy and fine wirebonding and direct component attachment.
The present invention overcomes the deficiencies of prior art approaches by providing a method for making an electronic circuit assembly comprising the steps of: (a) providing a specially designed tri-metal-layer precircuit, (b) selectively etching the precircuit so as to urge the formation of a particular type of undercut in the tri-metal precircuit structure, and (c) continuing to etch the precircuit until a finished circuit is formed, wherein the circuit includes circuit traces, air bridge crossovers, and base pads having one or more etched bumps thereon. These bumps may then be used to facilitate heavy and fine wirebonding and direct chip attachment.
It is an object and advantage that the present invention provides the aforementioned air bridge crossover circuit structure while also providing bumps specially etched from the precircuit structure which may be advantageously utilized to accommodate heavy and fine wirebonding and direct chip attachment.
Another advantage is that the bumps provided by the present invention may be used to assure a minimum solder joint standoff height before, during, and after reflow soldering of a surface mount electronic component.
Yet another advantage is that the bumps of the present invention may be arranged so as to retard solder joint crack propagation in reflowed electronic components.